The present invention relates to an optical functional semiconductor element for controlling an optical signal with light which is indispensable to fast optical signal processing in the fields of optical switching and optical information processing.
It is now expected that wide-band, new services which utilize fiber optic communication capable of super wide band, ultrafast transmission, such as picture communication and video distribution, will become widespread. In this instance, ultrafast signal processing is indispensable at nodes to which wide-band signals are concentrated. Attention is now focused on what is called an optical switching, optical signal processing system as a system which meets the above requirement. The optical switching, optical signal processing system is expected to permit faster switching operation than in the case of processing optical signals with an LSI or similar electronic circuit after converting them to electric signals and directly processes the optical signals, considered to allow further reduction of the processing time through parallel processing, or performs processing which makes the most use of properties of light.
One of important function in optical signal processing is a signal identifying capability that identifies an input signal for switching it to a desired route. Usually, a digital sequence of signals uses bit patterns for signal identification; the signal identifying capability is to perform a bit pattern matching operation. The bit pattern matching means a function which makes a check to see whether digital signal bits assigned to respective time slots of a plurality of input signal cells match or not and finally determines whether or not the plurality of cells are identical. The implementation of such a function requires an optical device which performs an exclusive OR (XOR) or exclusive NOR (XNOR) operation as a logical operation to provide a different output signal, depending on whether the plurality of bits are both zeros or ones.
FIG. 16 shows a conventional XOR optical logic element utilizing an optical feedback scheme. Reference numerals 100 and 100' denote phototransistors (HPTs) each composed of n-InP, p-InGaAsP and n-InP layers, and 101 and 101' LEDs each composed of n-InP, InGaAsP and p-InP layers. The LEDs are each connected to one of the two HPTs of the two pairs which are simultaneously irradiated with input light A and input light B, and two units, each consisting of the HPT connected to the LED and the HPT not connected thereto, are connected in parallel to the power supply. In FIG. 17 there is shown the sectional structure of the unit cell surrounded by the one-dot chain line. Reference numeral 102 denotes a semi-insulating InP layer, 103 an n-InP layer, 104 a p-InGaAsP layer, 105 an n-InP layer, 106 an InGaAsP layer, 107 a p-InP layer, 108 a p-InP layer, 109 an Au-Zu layer, 110 an Au-Sn layer, 111 a polyimide layer, and 112 a Ti/Au layer. The InGaAsP layers 104 and 106 correspond to the base layer of each HPT and the light emitting layer of each LED, respectively. If now the input light A (or B) is provided singly as depicted in FIG. 17, the HPT 100 (or 100') is turned ON, and consequently, the HPT 100' (or 100) connected thereto is turned OFF, allowing only the LED 101 (or 101') to emit light. On the other hand, when the input light A and the input light B are both provided, only those HPTs which are not connected to the LEDs 101 and 101' are turned ON; hence, the LEDs draw no current and do not emit light. When both the input light A and the input light B are not provided, no current flows to anywhere and neither of the LEDs does not emit light. From the above-described operations, it will be seen that the input light A and the input light B into the HPTs and the sum of output light C and output light D from the LEDs bear a relationship just like an exclusive OR (XOR).
FIG. 18 shows a development of the Applicants. The figure illustrates an XOR optical logic element employing a resonant-tunneling diode. There is also shown a band diagram of the optical functional semiconductor element for operation in the 1 .mu.m wavelength range which uses semiconductor materials of the InGaAsP/InGaAlAs series. Reference numeral 201 denotes an n-InP layer, 202 an InGaAsP active layer (the energy gap wavelength .lambda.g.about.1.55 .mu.m) which has a PN junction in its one interface and is capable of emitting light or modulating the transmittance by the injection thereinto of carriers, 203 an n-InP layer, 204 an i-InGaAs light absorbing layer (.lambda.g.about.1.65 .mu.m) which is capable of generating electron and hole carriers by absorbing light of a particular wavelength, 205 an i-InGaAs tunnel barrier layer, 206 a strained InGaAs or strained InAs quantum well layer, 207 an i-InAlAs tunnel barrier layer, 208 a p-InGaAs layer, 209 an i-InAlAs layer and 210 an InP layer. The layers 205, 206 and 207 form what is called a resonant-tunneling diode (RTD) 211, and the layers 204, 208 and 209 form what is called a triangular barrier diode (TBD) 212. The RDT 211 is provided in one of the i-layers of the TBD 212. Incidentally, the broken line indicates the Fermi-level. In this element, only when irradiated with light of an appropriate intensity, the active layer 202 is put in the ON state in which it is high-gain and is capable of emitting light or performing lasing, and when the incident light is absent or has an excessively high intensity, the layer 202 is put in the OFF state in which it is high-loss and hardly emits light. Thus, this element is an optical functional element which permits direct control of an optical signal through irradiation with light. In this instance, the high light sensitivity of the TBD 212 provides a high-efficiency, high-contrast characteristic even if the quantity of incident light is very small. Moreover, a bistable state can be achieved in the input/output characteristic.
The optical functional element which performs XOR and other logical operation through utilization of the conventional optical feedback scheme, as described above, converts the optical signal by a phototransistor to an electric signal and then drives a light emitting diode of low operating speed; hence, this element has the shortcoming of low operating speed. Furthermore, it is disadvantageous in that its construction and manufacturing process are complicated. In the optical XOR logical element using the RTD, since the resonant-tunneling diode is provided in the i-type layer which is exposed to an internal electric field, the negative resistance characteristic inherent in the resonant-tunneling diode is likely to become indistinct, making it difficult to perform a high-contrast optical logical operation; furthermore, a potential barrier impedes carriers generated in the light absorbing layer from flowing when exposed to an electric field, resulting in the retardation of the optical response operation.